Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013 Segment	Jun. 30, 2012	Dec. 31, 2012
Segment Information [Abstract]
Number of reportable operating segments defined by geographical regions			3
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	$ 150,170	$ 129,765	$ 296,565	$ 255,238
Net operating income	101,433	87,348	201,064	172,656
Management and other fees	3,234	2,796	6,182	5,240
Depreciation	(48,031)	(41,801)	(95,175)	(82,535)
General and administrative	(6,224)	(5,764)	(12,850)	(11,164)
Cost of management and other fees	(1,733)	(1,611)	(3,434)	(3,251)
Interest expense before amortization	(26,325)	(24,659)	(51,536)	(49,316)
Amortization expense	(3,002)	(2,882)	(5,932)	(5,754)
Interest and other income	1,917	5,455	6,939	7,868
Equity income from co-investments	7,282	3,111	11,493	5,451
Gain (loss) on early retirement of debt	1,024	(1,450)	1,024	(1,450)
Gain on sale of land	0	0	1,503	0
Gain on remeasurement of co-investment	0	21,947	0	21,947
Income from continuing operations	29,575	42,490	59,278	59,692
Net reportable operating segment - real estate assets	4,074,937		4,074,937		3,952,155
Real estate under development	70,242		70,242		66,851
Co-investments	618,259		618,259		571,345
Cash and cash equivalents, including restricted cash	45,384		45,384		42,126
Marketable securities	88,734		88,734		92,713
Notes and other receivables	110,544		110,544		66,163
Other non-segment assets	50,898		50,898		55,870
Total assets	5,058,998		5,058,998		4,847,223
Southern California [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	66,232	61,356	132,061	119,916
Net operating income	44,464	41,106	88,897	80,937
Net reportable operating segment - real estate assets	1,654,746		1,654,746		1,675,265
Northern California [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	53,089	42,570	103,859	84,192
Net operating income	36,462	29,226	71,746	58,168
Net reportable operating segment - real estate assets	1,625,567		1,625,567		1,489,095
Seattle Metro [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	27,081	22,668	53,408	44,977
Net operating income	17,792	14,841	35,164	29,673
Net reportable operating segment - real estate assets	707,473		707,473		699,465
Other Real Estate Assets [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property revenues	3,768	3,171	7,237	6,153
Net operating income	2,715	2,175	5,257	3,878
Net reportable operating segment - real estate assets	$ 87,151		$ 87,151		$ 88,330